1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator using a resonator such as a crystal, and a temperature compensation method for an oscillator.
2. Description of Related Art
A crystal oscillator is used as an oscillation element for obtaining a reference frequency in communication with a mobile phone, a GPS device or the like.
The crystal oscillator has a very high accuracy of oscillation frequency but it still has a slight temperature characteristic, and a usual AT-cut crystal oscillator shows a cubic curve.
An oscillator in which temperature changes are compensated by controlling the cubic curve to be cancelled is called a temperature-compensated crystal oscillator (TCXO).
There are two types of temperature compensation methods, i.e., an analog temperature compensation method and an LSI temperature compensation method, and the LSI temperature compensation method is the mainstream method because it has an advantage of reducing the size.
An LSI includes a temperature compensation circuit and an oscillation circuit. As the temperature compensation circuit, there have been proposed various methods, for example, the temperature compensation circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2002-198736.
The most representative temperature compensation circuit employs a method in which a voltage represented by a cubic curve is generated in an LSI to control an oscillation frequency of a voltage controlled crystal oscillator (VCXO).
In order to generate the cubic curve, the LSI has a thermometer (a first voltage) and combines a DC voltage (zero-order voltage), a secondary voltage and a tertiary voltage generated by a square-root circuit and a cube-root circuit. By using coefficients at the time of combining the voltages as parameters, the cubic curves can be freely drawn as shown in FIG. 1.
This is expressed as a following equation (1).V=A(T−T0)3+B(T−T0)2+C(T−T0)+D  (1)
Until now, the temperature characteristic of the crystal oscillator can be relatively accurately compensated by adjusting these four parameters of A, B, C and D in the Equation 1.